I assume that while his body could move at the speed of light, his reactions are still limited by the speed of his own thought. His body can move fast enough, but if he can't think fast enough, he's not going to be able to get out of the way.
The only reason Logia's seem to be immune to bullets normally is because they're bodies apparently dissolve automatically.
Hmm, that almost seems like a plot hole at first. But when you think about it, shouldn't Brook die again if he falls in the ocean? If the fruit brought him back to life, and the sea negates the fruit then....On the other hand, we also know a devils fruit user doesn't become immediately de-fruited upon contact with the sea. For example, Luffy was trapped underwater during the Arlong arc, yet the villagers could still pull his head out of the water cause of his elasticity